This invention relates to metallized plastic film capacitors suitable for the application of voltages above 240 volts a-c. More particularly, this invention relates to a fluid dielectric for a-c capacitors in which the imbibition of such fluid into the dielectric plastic film of such capacitors is reduced over dielectric fluids such as mineral oil, dioctyl phthalate, diisononyl phthalate and the like.
During the manufacture of a high voltage a-c metallized film capacitor, air spaces or pockets may be formed between the surfaces of a dielectric plastic film and overlying metallized electrodes of the capacitor body as well as between the body and the inner surface of the housing for the capacitor body. Air in such pockets may include moisture. Air pockets and moisture are enemies of the plastic film and of the metallized electrodes. The presence of air pockets and moisture between the dielectric plastic film and the metallized electrodes may establish a condition where if the field strength of air or of the air-moisture combination is exceeded, disruptive electrical or corona discharges occur. Such discharges may have sufficient electrical energy associated therewith to cause erosion of the metallized film and/or gas ionization to cause harmful degradation of the plastic film. Once high energy corona discharges and gas ionization start, no material can long endure the bombardment. Harmful erosion of the metallized electrodes and/or degradation of the plastic film result in early capacitor failure.
Generally, vacuum drying is used to help eliminate moisture and to help remove air from pockets in and around the capacitor body. To help prevent air and moisture from thereafter contacting the capacitor body, high voltage metallized film capacitor manufacturers use a dielectric fluid to impregnate and surround the capacitor body. Impregnation of the capacitor body usually takes place through the limited space between the plastic film and the closely adjacent electrode. However, impregnating fluid may be imbided into the plastic film in such a way as to cause harmful swelling of the film. Harmful swelling of the film causes it to soften which may result in high energy electrical discharges which cause erosion of the electrodes and degradation of the plastic film resulting in early capacitor failure. Difficulties associated with harmful imbibition of dielectric fluids such as mineral oil, dioctyl phthalate, diisononyl phthalate and the like into a plastic film such as biaxially oriented polypropylene can be reduced by using the dielectric fluid of this invention.